Method and device for the liquefaction of a gaseous co2 stream

ABSTRACT

The invention relates to a device for the liquefaction of a gas flow containing at least 95 mol-% carbon dioxide, comprising at least a first compression stage in which the feed gas flow is compressed; means for condensing the compressed flow, formed by two in-series phase separators, in order to partially condense the flow and produce a liquid flow; a first heat exchanger; a second heat exchanger; a means for conveying at least part of the liquid flow into the tubes of the first heat exchanger; a means for discharging a first part of the liquid cooled in the second heat exchanger, a valve, a means for conveying as second part of the liquid cooled in the second exchanger, and a means for compressing at least part of the vaporized flow and mixing this with the fed gas flow.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT Application PCT/FR2015/050049 filed Jan. 9, 2015 which claims priority to French Patent Application No. 1450201 filed Jan. 10, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process and to a device for the liquefaction of a gaseous CO₂ stream. The stream contains at least 95 mol % of CO₂, indeed even at least 99 mol % of CO₂.

SUMMARY

The invention consists of a process which makes it possible to liquefy a CO₂ stream containing impurities (for example H₂ or N₂).

It is known from JP-A-64084087 to liquefy a flow predominantly containing CO₂ by drying the flow to be liquefied in a dryer, by cooling it in order to partially liquefy it, by sending it to a first phase separator, by sending the liquid from the first phase separator to a second phase separator and by extracting the liquefied flow from the second phase separator. The gas from the second phase separator is reheated and sent upstream of the dryer.

This process does not make it possible to subcool the liquid produced, which proves to be of use when the liquid has to be used at a lower pressure than the liquefaction pressure.

One aim of the present invention is to overcome the failings of the prior art. According to a subject-matter of the invention, there is provided a process for of a gas flow containing at least 95 mol %, indeed even at least 99 mol %, of carbon dioxide, in which:

a) the feed gas flow is compressed in at least a first compression stage,

b) the compressed flow is cooled in order to at least partially condense it in order to produce a liquid flow, by being cooled in a first heat exchanger other than the first heat exchanger in order to partially condense it, the partially condensed flow is reduced in pressure and sent to a first phase separator, a liquid from the first phase separator is reduced in pressure and then sent to a second phase separator and the liquid flow is withdrawn from the second phase separator,

c) at least a portion of the liquid flow is cooled in the tubes of a second heat exchanger which is a shell and tube heat exchanger,

d) a first part of the liquid cooled in the second exchanger, subsequently reduced in pressure, acts as liquid product,

e) a second part of the liquid cooled in the second exchanger or of a liquid produced by reducing in pressure and partially vaporizing this liquid is reduced in pressure in a valve and is evaporated in the shell of the first exchanger in order to form a vaporized flow, and

f) at least a part of the vaporized flow is compressed and mixed with the feed gas flow.

According to other optional aspects:

-   -   a gas from the first phase separator is mixed with the feed gas         flow at a first pressure,     -   a gas from the second phase separator is mixed with the feed gas         flow,     -   the gas from the second phase separator is mixed with the feed         gas flow at a second pressure lower than the first pressure,     -   the gas from the second phase separator is mixed with the feed         gas flow upstream of the first compression stage,     -   the liquid from the first phase separator is not cooled upstream         of the reduction in pressure,     -   the second exchanger is a final subcooler,     -   the final product is not cooled downstream of the second         exchanger,     -   no fraction of the liquid product is recycled to the first         compression stage,     -   the second part of the liquid cooled in the second exchanger is         reduced in pressure in a valve and is reheated solely in the         second heat exchanger in order to form a vaporized flow.

According to another subject matter of the invention, there is provided a device for the liquefaction of a gas flow containing at least 95 mol %, indeed even at least 99 mol %, of carbon dioxide, comprising at least a first compression stage in which the feed gas flow is compressed, means for condensing the compressed flow, in order to partially condense it in order to produce a liquid flow, a first heat exchanger, in which the compressed flow is cooled in order to partially condense it, means for reducing in pressure the partially condensed flow, a first phase separator into which the pressure-reduced flow is sent, means for reducing in pressure a liquid from a first phase separator, a second phase separator, means for sending the pressure-reduced liquid to the second phase separator and means for withdrawing the liquid flow from the second phase separator, a second heat exchanger (9) which is a shell and tube heat exchanger, means for sending at least a part of the liquid flow into the tubes of the second heat exchanger, means for taking out, as liquid product, i) a first part of the liquid cooled in the second exchanger, subsequently reduced in pressure, or ii) a first part of a liquid produced by reducing in pressure and by partially vaporizing the liquid cooled in the second exchanger, a valve, means for sending a second part of the liquid cooled in the second exchanger or of a liquid produced by reducing in pressure and partially vaporizing this liquid to be reduced in pressure in the valve and to be vaporized in the shell of the second exchanger, in order to form a vaporized flow, and means for compressing at least a part of the vaporized flow and mixing it with the feed gas flow.

According to other optional aspects, the device:

-   -   comprises means for sending a gas from the first phase separator         downstream of the first compression stage,     -   comprises means for sending a gas from the second phase         separator upstream of the first compression stage,     -   does not comprise cooling means between the first phase         separator and the valve.

The second heat exchanger is preferably the final subcooler of the device.

The invention will be described in a more detailed manner with reference to the FIGURE.

BRIEF DESCRIPTION OF THE DRAWING

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawing, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 illustrates a schematic representation of one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, the process for the liquefaction of a flow 1 containing at least 95 mol %, indeed even at least 99 mol %, of carbon dioxide is carried out by cooling by indirect heat exchange with a cold source. The feeding with CO₂, as a function of its pressure, is carried out at interstage on a cycle compressor 3, between stages 3B and 3C.

The final two stages 3C, 3D of this compressor 3 compress the flow 1 until a sufficient pressure is reached which makes it possible to condense the gas stream confronted with a cold source 5 available on site (for example ice-cold water) in a heat exchanger 7.

The CO₂ thus condensed at high pressure will undergo a succession of reductions in pressure in valves V1, V2 in order to be selfcooled by generation of a gas.

The first reduction in pressure in the valve V1 will preferably take place at the inlet pressure of the final wheel 3D of the cycle compressor 3. Thus, the gas 4 generated subsequent to the reduction in pressure of the liquid at the pressure-reduced equilibrium originating from the phase separator S1 can be recycled upstream of the final wheel of the cycle compressor.

A second stage of reduction in pressure of the liquid from the phase separator S1 in a second valve V2 is preferably envisaged in order to reduce the pressure of the liquefied CO₂, before entering the main exchanger 9, thus making possible a CAPEX saving on this same exchanger. Here again, the pressure-reduction pressure is chosen in order to make possible the recycling of a gas 6 from a second phase separator S2 upstream of the penultimate compression wheel 3C.

This sequence of reductions in pressure in the valves V1, V2 makes it possible to cool the liquefied CO₂ 18 while limiting the OPEX impact by recycling as much as possible in the final compression stages.

Once partially reduced in pressure and cooled, the stream of liquid CO₂ 18 will enter a heat exchanger 9 in order to be strongly subcooled therein. After subcooling, the liquid 18 is divided into two parts. The part 11 is reduced in pressure in a valve V3 in order to form a liquid product at a pressure required by the client, typically 7 bara. A part 13 is evaporated against the liquid 18 in the heat exchanger 9, after reduction in pressure in a valve V4. The reduction in pressure in the valve V4 brings the liquid up to reaching a temperature as close as possible to that of the triple point (−56.5° C.).

The vaporized low-pressure CO₂ 15 is subsequently recycled to the first stages 3A, 3B of the cycle compressor 3 in order to ensure a liquefaction yield of 100%. It is mixed with the flow 1 between stages 3B and 3.

The heat exchanger 9 mentioned above will be an exchanger of shell and tube type, with the flow 18 to be cooled in the tubes and the liquid 13, reduced in pressure to a pressure approximately that of the triple point, in the shell, in order to avoid any risk of accident, as a result of a possible icing up of this same stream (in particular in the case where the cycle compressor 3, to which the vaporized liquid 15 will return, sucks too much and causes the pressure to fall below that of the triple point of CO₂). 

1.-13. (canceled)
 14. A process for the liquefaction of a gas flow containing at least 95 mol % of carbon dioxide, comprising: a) compressing the feed gas flow in at least a first compression stage, b) cooling the compressed flow in order to at least partially condense the compressed flow, thereby producing a liquid flow, the cooling being performed in a first heat exchanger in order to partially condense it, reducing the partially condensed flow in pressure thereby producing a first reduced pressure flow and sending the first reduced pressure flow to a first phase separator, reducing a liquid from the first phase separator in pressure thereby producing a second reduced pressure flow and then sending the second reduced pressure flow to a second phase separator and withdrawing the liquid flow from the second phase separator, c) cooling at least a portion of the liquid flow removed from the second phase separator in the tubes of a second heat exchanger which is a shell and tube heat exchanger, d) cooling a first part of the liquid cooled in the second exchanger, subsequently reducing the cooled first part in pressure, thereby producing a liquid product, e) cooling a second part of the liquid cooled in the second exchanger or of a liquid produced by reducing in pressure and partially vaporizing the cooled second part by reducing the pressure in a valve and is evaporating the cooled second part in the shell of the first exchanger in order to form a vaporized flow, and f) compressing at least a part of the vaporized flow and mixing the compressed vaporized flow with the feed gas flow.
 15. The process of claim 14, in which a gas from the first phase separator is mixed with the feed gas flow at a first pressure.
 16. The process of claim 14, wherein a gas from the second phase separator is mixed with the feed gas flow.
 17. The process of claim 16, wherein the gas from the second phase separator is mixed with the feed gas flow at a second pressure lower than the first pressure.
 18. The process of claim 17, wherein the gas from the second phase separator is mixed with the feed gas flow upstream of the first compression stage.
 19. The process of claim 14, wherein the liquid from the first phase separator is not cooled upstream of the reduction in pressure.
 20. The process of claim 14, wherein the second heat exchanger is a final subcooler.
 21. The process of claim 20, wherein the final product is not cooled downstream of the second exchanger.
 22. A device for the liquefaction of a gas flow containing at least 95 mol % of carbon dioxide, comprising at least a first compression stage in which the feed gas flow is compressed, a means for condensing the compressed flow, in order to partially condense it in order to produce a liquid flow, a first heat exchanger, in which the compressed flow is cooled in order to partially condense it, a means for reducing in pressure the partially condensed flow, a first phase separator into which the pressure-reduced flow is sent, a means for reducing in pressure a liquid from a first phase separator, a second phase separator, a means for sending the pressure-reduced liquid to the second phase separator and a means for withdrawing the liquid flow from the second phase separator, a second heat exchanger which is a shell and tube heat exchanger, a means for sending at least a part of the liquid flow into the tubes of the second heat exchanger, a means for taking out, as liquid product, a first part of the liquid cooled in the second exchanger, which is subsequently reduced in pressure, a valve, a means for sending a second part of the liquid cooled in the second exchanger to be reduced in pressure in the valve and to be vaporized in the shell of the second exchanger, in order to form a vaporized flow, and a means for compressing at least a part of the vaporized flow and mixing it with the feed gas flow.
 23. The device of claim 22, further comprising a means for sending a gas from a first phase separator downstream of the first compression stage.
 24. The device of claim 22, further comprising means for sending a gas from the second phase separator upstream of the first compression stage.
 25. The device of claim 22, not comprising cooling means between the first phase separator and the valve.
 26. The device of claim 22, wherein the second heat exchanger is the final subcooler of the device. 